At 12:49 PM 1/10/02 -0600, Rocky Frisco wrote:
>"Any advice about placing an MGB engine and transmission in a Mk II MGA? .... Am I right in assuming I will need to make up motormounts and possibly an odd-length drive-shaft?"

The earlier the MGB parts the easier is the job.

Any B-series engine can be a bolt-in, with the right combination of parts, when mated to the original MGA 3-syncro gearbox. There are a lot of MGA 1800's running around with either 3-main or 5-main engines. Just not to many MGA's with 4-syncro gearboxes.

You need to use the engine rear plate that fits the front of the gearbox. To attach a 5-main-bearing engine to the 3-syncro gearbox you use the rear plate (carries the rear seal) and flywheel (small flywheel) and starter motor (inertia starter) from the 64-67 MGB GB engine. For the MGA 1500 type gearbox with the lower position for the starter motor you would have to use the MGA 1500 engine rear plate, and you have to bore out the center hole to fit the rear rubber seal for the 5-main bering crankshaft. Also use the '65-'67 electric tachometer, as the 5-main-bearing engine does not have a mechanical tach drive, and the early electric tach has the same face appearance as the MGA and early MGB mechanical tach. 1965-1967 was a transition period when the first 5-main engines were used with a small flywheel while still being mated to the earlier 3-syncro gearbox. Also see article on the MGB clutch transplant.


MGA 1500 rear plate with low starter position, modified to accept rear seal for 1800 five main bearing engine. The center hole is bored out to accept the rubber seal. Also notice the four tapped holes added for screws to mount the MGB seal retainer. I will not speculate about why the factory thought this part was necessary.

(Thanks to Roy Dougherty for these pictures).


Front side of rear plate after machining.

The shallow slot at the bottom of the big bore is the original drain slot in the front of the plate to allow oil that has sneaked past the mechanical seal (3 main bearing engine) to drain out between the engine block and the rear plate before it gets back to the flywheel and clutch area. During final assembly of the newly modified parts you will be plugging up that shallow slot with silicone sealant so no oil can escape from the engine. A more permanent fix would be to weld up the slot and machine the face of the plate flat.

For post 1975 engines you change the engine front plate to the earlier type so you can use the earlier rectangular engine mounts. You need to use one of the short nose MGB water pumps to avoid interference with the MGA radiator. Also use the matching MGB pulley to position the belt in the correct plane (small difference from MGA). If the fan looks too close to the radiator it may be prudent to shim the radiator forward about 1/4 inch. Use a clutch plate that matches the splines on the gearbox input shaft. Use an adapter bushing for the spigot bearing in the back end of the crankshaft.

Internally the MGB 3-syncro gearbox is virtually identical to the MGA gearbox, with all of the parts between the front and center main ball bearings being interchangeable. The noticeable differences are the input shaft where the splines and pilot diameter are different (but adaptable), and a bit of a difference in the positioning of the manual shift lever. Most significant is a difference in the gearbox rear mount, where the MGA has a latteral rubber mount with a single bolt, and the MGB uses small rectangular rubber mounts. Most people end up cutting the MGA frame and fabricating a removeable cross member for the MGB gearbox mount. The MGA 1600 gearbox rear housing can bolt up directly to the early MGB three syncro gearbox, effectively converting it to a MGA MK-II unit which will bolt it without any modification to the frame. This also allows use of the late production MGB 3-sync box (last half of 1967 production year) with the slightly larger layshaft with four oil holes and four needle bearings.

Fitting the later MGB 4-syncro gearbox into the MGA is more of a PITA, mostly because of the larger bellhousing. I have seen this done, but the bellhousing is a very tight fit in the tunnel (without modification of the tunnel), such that it appears impossible to R&R the engine, unless you have a removeable cross member under the frame for the gearbox mount. But like they say, where there's a will there's way. In any case, use the driveshaft that goes with the gearbox you're using.

A more difficult combination would be trying to mate a 3-main-bearing engine to a 4-syncro gearbox. Problem here is the small pilot diameter in the back of the crankshaft, and the larger pilot on the gearbox input shaft. You cannot swap input shafts between the 3-syncro and 4-syncro units, so you have to have the pilot nose of the input shaft machined down to fit the bushing in the crankshaft. I also do not know personally if there is any engine rear plate that would work with this combination. I believe the standard plate for 5-main engine and the 4-syncro gearbox will work if you counterbore the bolt holes similar to the MGA plates so the bolt heads will clear the earlier flywheel.

Addendum 23 Aug 05:
European Car magazine ran a full article on how to convert your MGA to 1800 power the correct way. It also list part numbers and suppliers. You might contact them to see if a back issue is available.
European Car Magazine, April 1993, Volume 24, Number 4
The article starts on page 35 then finishes on page 118/119.